Sound attenuating device



Aug. 23, 1938. R. B. BOURNE i SOUND ATTENUATING DEVICE Filed Oct. 25,1935 MJMQBMQ. RKKY n @a lr- I/ INVENTOR R0/.M0 AZ300/PMS J- BY l ToR'NEYs Patented Aug. 23, 1938 UNITED STATES PATENT OFFICE SOUNDATTENUATING DEVICE Roland B. Bourne, Hartford, Conn., assignor to TheMaxim Silencer Company,

Hartford,

4 Claims.

The present invention relates to improvements in sound attenuatingdevices employing reactive acoustic sidebranches acoustically coupled toa main sound conducting channel.

The types of acoustic sidebranches employed in the several embodimentsof the invention are classed as linear sidebranches, wherein change ofacoustic phase takes place as a function of distance and wherein thelength of the sidebranch is ordinarily a principal factor in determiningits resonance frequencies. The invention makes use of various types oflinear sidebranches which may be acoustically coupled to a rnain soundconducting channel or other enclosure. This acoustical coupling may beaccomplished in different manners.

A characteristic of linear resonators in general is the fact that theyhave a plurality of modes of vibration, the natural frequencies forminga series. In the case of a linear resonator of ccnstant cross sectionalarea throughout its length, and having unrestricted openings at its endor ends, and in the case of a complete cone entirely open at its baseand closed at its vertex, these natural frequencies form an integralseries. In the case of certain other types of linear resonators havingunrestricted openings, the natural frequencies do not form an integralseries.

A linear resonator acoustically coupled to a main sound conductingchannel offers attenuation to sound waves, in the channel, offrequencies directly dependent upon the natural frequencies of theresonator. It is obvious that a resonator having an integral series ofresponse frequencies may be used to attenuate a sound Wave having anintegral series of component frequencies. Two or more resonators may beassociated with a main sound conducting channel in such a way thatsubstantially continuous attenuation over a wide range of frequenciesmay be obtained. 'Ihe sound attenuating device thereby resulting maytake the form of a sound conducting channel having a system of discreteresonators acoustically coupled to it or it may take the form of anacoustic Wave lter having recurrent pass and attenuation bands or theremay result various combinations whereby a desired acoustic result isobtained.

Under certain conditions, the length of acoustlc sidebranches of thelinear type may be a disadvantage from the viewpoint of available spacefor an installation. This is especially true in the case where theobjectionable sound Waves are of extremely low frequency. Any means forap- L preciably altering its lowest response frequency is desirable.This is one of the main purposes of the invention.

Another important purpose of the invention is to provide means forconverting a non-integral series of response frequencies of a lineartype resonator into a substantially integral series. A further purposeof the invention is to provide means whereby an acoustic sidebranch maybe designed having a non-integral series of response frequencies so thatattenuation may be offered to corresponding non-integrally related soundfrequencies passed by an acoustic wave lter or other selective soundattenuating system. Further purposes and objects of the invention willbe disclosed as the specification proceeds.

The purposes of the invention are accomplished by the use of arestricted opening or its equivalent at the mouth of the linear acousticsidebranch, at the point where the sidebranch is acoustically coupled tothe main sound conducting channel or other enclosure whereinobjectionable sound waves may occur.

The use of a restricted opening at the mouth of a linear resonator,which is at least a quarter wave length long, is not comparable to theuse of a restricted opening giving into a volumetric or Helmholtz typeresonator, which is small compared to a wave length. In the latter case,the acoustic conductivity of the opening is a principal factor indetermining the resonance frequency of the resonator, the frequencyvarying directly as the square root of the conductivity of the opening.The volumetric resonator, being of small dimensions relative to a wavelength of sound for its resonance frequency, does not depend upon anyparticular dimension for computing its acoustical capacitance or volume.For instance, if of cylindrical shape, the volume and therefore thecapacitance of the resonator might be held constant whilst varying boththe length and diameter appropriately, the resonance frequency remainingconstant for a given medium and a given conductivity. In the case of alinear resonator of cylindrical shape, however, the volume may be, forinstance, doubled, Without appreciably changing the fundamentalfrequency of response as long as the length is unaltered. The acousticsystem comprising a linear resonator with a restricted coupling openingmay be said to be a concentrated or lumped inertance in series with adistributed inertance and capacitance.

In the design of linear resonators used as acoustic sidebranches, theconductivity of the mouth has been generally taken into account byassuming that the sidebranch extends a certain distance into the channelor enclosure to which it is coupled. This procedure results in aneffective increase, generally quite small, in the length of thesidebranch, the ratio between the various modes of vibration remainingunchanged by the process.

In my invention, however, the use of a substantially restricted openingrbetween the linear resonator proper and the main sound conductingchannel not only increases the acoustic length of the sidebranch by avery appreciable amount, but also materially alters the relation betweenthe fundamental or lowest response frequency and the several overtones,Whether these overtones be harmonic or not. There is a practical limitto the extent that the conductivity may be reduced. In the variousembodiments of my invention, the conductivity is formed by the mosteiiicient orifice possible, so that no appreciable additional amount ofacoustic resistance due to surface friction, eddies, narrow openings andthe like is introduced. Therefore a preferred type of opening to formthe necessary conductivity is a single round hole in a flat plate. Insuch embodiments as require slot-like conductivities, such openings aredisposed in centrally located main conducting channels rather than` inannular channels having large peripheries and therefore higher acousticresistance. The acoustic efficiency of a sidebranch is impaired by theintroduction of acoustic resistance thereinto.

The invention finds useful application in sound attentuating devices orsilencers for use in conjunction with internal combustion engine eX-hausts, air intakes, discharges and the like. In commercial embodiments,the over-all length of the silencer is directly influenced, in manycases, by the lengths of the acoustic sidebranches, since they areusually disposed parallel to the axis of the main channel through thedevice; and therefore, substantial diminution of the lengths of thesidebranches is reiiected directly in the increased commercialadaptation of the silencer due to its shorter length.

Inasmuch as some of the embodiments herein shown employ acousticsidebranches of non-uniform cross sectional area as a function ofdistance along their lengths, I refer to my Patents 2,017,745;2,017,746; 2,017,747; and 2,017,748, granted October 15, 1935, whereinseveral such acoustic sidebranches are shown.

Referring to the drawing,

Fig. 1 shows a cylindrical resonator embodying the principle of theinvention;

Fig. 2 shows a conical resonator embodying the principle of theinvention;

Fig. 3 shows a truncated conical resonator embodying the principle ofthe invention;

Fig. 4 shows a sound attenuating device illustrating an application ofthe invention;

Fig. 5 shows a graph germane to Figs. 4 and 6;

Fig. 6 shows a composite sound attenuating device embodying theprinciple of the invention; and

Fig. 7 shows a simple sound attenuating device employing the resonatorsof Fig. l and Fig. 2.

The resonator of Fig. l comprises the cylindrical casing I, of uniformcross sectional area throughout its length, closed at one end by thetransverse header 2 and fitted at the other end by the transverse plate3 having a centrally disposed opening 4 forming an acoustic conductivityCo. This discussion is limited to embodiments wherein the length L ofthe resonator is appreciably greater than its diameter D and wherein thelength L is an appreciable fraction of a wavelength for the lowest soundfrequency to which the resonator resonates. If L is equal to D, forinstance, the device functions as a volumetric or Helmholtz resonator.While there is no sharp line of demarcation between the Helmholtz andso-called linear resonator, the resonators involved in my invention aresufficiently long as to be adequately treated on the basis ofdistributed acoustic elements of inertance and capacitance rather thanon the lumped theory, which is applicable to the Helmholtz type. Theresonator of Fig. 1, with the end plate 3 removed has responsefrequencies determinable from wL Il Tf-.5, 1.5, 2.5, etawhere n 1s odd(1) ci: 21rx frequency L=acoustc length of the resonator including anend correction C=vclocity of sound in the medium The addition of theIend plate 3 with a suitable aperture 4 therein materially alters thisseries of response frequencies. A representative series thus obtained isgiven by relation has been converted into a non-integral one. In termsof the length of the device to respond to a given frequency, it is seenthat a saving of 20% is effected. Such a resonator, when used as anacoustic sidebranch acoustically coupled to a sound conducting channelwill offer attenuation to sound frequencies determinable in accordancewith an equation similar to Equation 2.

Fig. 2 shows a complete conical resonator of slant length L and diameterD. The cone 5 is fitted at its large end with the transverse end plate 6having a circular opening 1 therein to form a conductivity Co. Withoutthe end plate 6, the conical resonator has natural frequencies which maybe shown to be given by ;:=1, 2, 3, 4, 5, etc. (3)

Such resonators are discussed in my patents above referred to. Theaddition of the end plate 6 with its acoustic conductivity Co alters theabove series of response frequencies. A representative series is shownby It will be seen that the effect of the conductivity Cu in the case ofthe cone is very similar to its effect in the case of the cylindricalresonator of Fig. 1.

Certain linear type resonators having unrestricted openings thereintohave response frequencies which do not bear an integral relation to eachother. An example of a linear type resonator having a non-integralseries of response frequencies is a truncated cone, open at its base andclosed at its small end by an imperforate header. Fig. 3 shows atruncated conical resonator of length L and diameter D comprising thetapered shell 8, closed at its small end by the n R-. 1.56, 2.54, 3.52,which approaches as a limit where n is an odd integer.

Equation (5) shows that this type of resonator yields av series ofresponse frequencies resembling those obtained for a closed cylinder ofuniform cross sectional area and having no restricted opening thereinto,as given by Equation (l), eX- cept that the fundamental frequency ishigher and succeeding overtones are also higher but by a decreasingamount approaching a limit for the nth overtone, Where the resonancefrequencies of the tWodevices coincide. In other words, for highovertones, the truncated cone behaves like a closed cylinder. By the useof the end plate II) having an aperture II therein, I have found that itis possible and readily practicable to choose a value for theconductivity C0, as represented by the aperture II, whereby the deviceyields substantially the same integral series of response frequencies as4does a closed, cylindrical resonator of uniform cross sectional areahaving no restricted opening at its mouth, said frequencies being givenby Equation (l) above. Different ratios between the diameters D and drequire different conductivities in order to bring the responsefrequencies into an integral series. For instance, for the case whereD=3d', the value of Co should be less than for the case where D=2d. Theconductivity Co is preferably formed by a single aperture, as shown,rather than a number of smaller apertures, in order to keep the acousticresistance ofthe opening low. The truncated conical resonator of Fig. 3could be used .as an acoustic sidebranch in association with a mainsound conducting channel to attenuate therein a series of soundfrequencies as given by Equation (1). There would result a saving intotal volume over a cylindrical sidebranch of the same frequencyresponse.

A simulation of the truncated cone of Fig. 3 is shown incorporated inthe device of Fig. 4. This device is a sound attenuator having entirelynovel features. It comprises the generally cylindrical casing I2, havingcentrally disposed openings I3, IlI at either end thereof, and a unitcoaxially mounted therewithin comprising the cylindrical shell I5, thecone I 6, having its open base afxed to one end of the cylinder i5 andextending inwardly to a point in adjacency to the other end of saidcylinder I5, whereby are formed the conical sidebranch i? and theconico-annular sidebranch I8. The open end of the cylinder l5 is fittedwith the transverse header I9 having a centrally disposed aperture 20therein. Both the conical sidebranch I'I and the conico-annularsidebranch i3 have an acoustic length L, as shown. These twosidebranches are acoustically coupled to opposite ends of an annularmain sound conducting channel M which is formed between the inside ofthe casing I2 and the outside of the cylinde-r I5. The sidebranch unitis, of course, made shorter than the casing I2 so that the main channel(and path for exhaust or other gases) extends from the opening I3 to theopening I4 as shown by successive radial and annular paths. I have foundthat it is readily possible to choose a value for the conductivity ofthe aperture 20 whereby the conico-annular sidebranch i8 offers peakattenuation to a series of sound frequencies which do not departsensibly from those given by Equation (l) above. Since, as has beenpointed out, the conical sidebranch I'I offers peak attenuation to soundfrequencies given by Equation (3), it is readily seen that the device ofFig. 4 offers peak attenuation to a series of sound frequencies given bymL lT-.5, 1.0, 1.5, 2.0, 2.5, etc.

These frequencies are the same as would be given by a single cone ofacoustic length 2L. In the latter case, however, the attenuation wouldbe theoretically zero for a series of intermediate frequencies, While inthe case of the device of Fig. 4 the attenuation does not fall to Zeroat any intermediate frequency. Fig. 5 shows a plot of measuredattenuation vs. frequency characteristics for a device built inaccordance with Fig. li, and having an acoustic length of L. It will beseen that the actual attenuation peaks occur at frequenciessubstantially in accordance with Equation (6) Another set of abscissaeare given for the case Where the acoustic length is %L, as discussedlater. A similar type of curve is obtainable for a cone and cylinderused as successive sidebranches along a main conducting channel, butsuch a device would be twice as long as that shown in Fig. 4. Thepractical utiiity of this embodiment of the invention is in providing asimple, cheap and effective attenuator for a simple sound havingovertones in harmonic relation.

A further useful purpose of the device of Fig. i is shown in Fig. 6wherein one unit of a composite sound attenuating device is designed tooffer attenuation to sound frequencies suffering little or noattenuation` in their passage through another part thereof. It comprisesthe generally cylindrical casing 2| divided by the transverse header 22into two sections, A and B, respectively. Section A comprises the mainsound conducting channel 23 formed by the tubular member 24 extendinglongitudinally therethrough and the closed cylindrical paired acousticsidebranches 25, 26, acoustically coupled to the main channel 23 throughthe slots 2'1, 28 respectively. The relative dimensions of thesidebranches with respect to the distance along the main channel betweenthem are shown in the figure. This device is completely described inUnited States Patent No. 1,910,672. This section A is an acoustic Wavefilter having a series of attenuation and pass bands. The pass bands areat values of These Values are seen to be in the proportion 1:2:324: etc.Consequently the device of Fig. 4 is readily adaptable for providingpeak attenuation to those sound frequencies represented by Equation (7).Accordingly, section B of Fig. 6 contains the sound attenuating unit 29of acoustic length %L, as shown. Data for Fig. 5 was taken With thedevice inserted in an acoustically long line and what is plotted is theinsertion, loss in decibels.

A sound attenuating device which provides peak attenuation for anon-linear series of sound frequencies in two overlappingcharacteristics is shown in Fig. '7. It comprises the generallycylindrical casing 30 divided into two compartments by the perforatetransverse header 3l. In one of said compartments is mounted the closedcylindrical sidebranch 32 constructed after the manner of Fig. 1. In theother of said compartments is mounted the conical acoustic sidebranch 33constructed after the manner of Fig. 2. In addition, a simple system ofbaffles 34 for additional high frequency attenuation is mountedexteriorly of the cone forming the conical sidebranch 33. Each of thetwo acoustic sidebranches contributes to the total attenuation in amanner similar to that indicated in Equations 2 and 4.

While I have shown but a few of the applications of the principle of myinvention to practical embodiments thereof, many more and usefulapplications may be made within the scope of this disclosure.

I claim:

1. A sidebranch structure adapted for incorporation in a soundconducting channel which comprises a cylindrical shell, a conical shelltelescoped wi. hin the cylindrical shell and joined thereto at one end,whereby a conical sidebranch is formed within the conical shell and aconico-annular sidebranch is formed between the conical and thecylindrical shells, both of said sidebranches being open at their largerends, and an apertured plate partially closing the opening into theconico-annular sidebranch.

2. A sidebranch structure adapted for incorporation in a soundconducting channel which comprises a Cylindrical shell, a conical shelltelescoped within the cylindrical shell and joined thereto at one end,whereby a conical sidebranch is formed Within the conical shell and aconicoannular sidebranch is formed between the conical and thecylindrical shells, both of said sidebranches being open at their largerends, and an apertured plate partially closing the opening into theconico-annular sidebranch, the opening in the apertured plate beingsuiliciently restricted to cause the conico-annular sidebranch torespond to a series of frequencies substantially intermediate those towhich the conical sidebranch responds.

3. An acoustic silencing device comprising a tubular member defining theouter boundary of a main sound conducting channel, a cylindrical membermounted Within the tubular member and spaced therefrom so as to definethroughout its length the inner boundary of the main sound conductingchannel, a conical member telescoped within the cylindrical member andjoined thereto at its larger end, whereby a conical sidebranch is formedwithin the conical shell and a conicoannular sidebranch is formedbetween the conical and the cylindrical shells, both of saidsidebranches being open at their larger ends, and an apertured platepartially closing the opening into the conico-annular sidebranch.

4. An acoustic silencing device comprising a tubular member defining theouter boundary of a main sound conducting channel, a cylindrical membermounted within the tubular member and spaced therefrom so as to deiinethroughout its length the inner boundary of the main sound conductingchannel, a conical member telescoped within the cylindrical member andjoined thereto at its larger end, whereby a conical sidebranch is formedwithin the conical shell and a conicoannular sidebranch is formedbetween the conical and the cylindrical shells, both of saidsidebranches being open at their larger ends, and an apertured platepartially closing the openingr into the conico-annular sidebranch, theopening in the apertured plate being suciently restricted to cause theconico-annular sidebranch to respond to a series of frequenciessubstantially intermediate those to which the conical sidebranchresponds.

ROLAND B. BOURNE.

Til

